Current furnace heat exchanger flames are long. For example the flame's length that can extend substantially through one or more passes of a heat conduction tube of the heat exchanger. As a result, the flame can distribute heat over a substantial length of the heat exchanger's heat conduction tube. In the presence of such a long flame, however, it is often not desirable to have intrusive heat transfer enhancements (e.g., dimples) because there is a significant danger of overheating the heat exchanger, or, of quenching the flame and producing carbon monoxide (CO). Consequently, the first pass of a furnace heat exchanger often runs cooler than desired, and thus represents inefficiently used surface area for heat exchange. Another consequence of using a long flame can be poor mixing between the fuel (e.g., natural gas or propane) and air. This, in turn, can result in CO or soot production under certain operating conditions.